Aqueous dispersions of anionic polymers are widely used in industry, for examples as flocculating agents, in particular as drainage and retention aids in papermaking and for water purification. In general, such dispersions comprise a water-soluble anionic dispersion polymer dispersed in an aqueous continuous phase which may contain one or more water-soluble salts and one or more stabilizers. The dispersions are generally prepared by dispersion polymerization in which the monomers and initiator are soluble in the polymerization medium, but the aqueous medium containing large amounts of water-soluble salts is a poor solvent for the resulting polymer. The reaction mixture is homogeneous at the onset, and the polymerization is initiated in a homogeneous solution. Phase separation occurs at an early stage of the polymerization process and colloidal polymer particles are formed and stabilized by adsorption of stabilizer.
WO 2001/18063 discloses a method of increasing retention and drainage in papermaking using high molecular weight water-soluble anionic or non-ionic dispersion polymers. The dispersion polymer is prepared using from about 2 to about 40 weight percent based on the total weight of the dispersion of a water-soluble salt.
WO 2006/123993 discloses a process for preparing a polymer dispersion and a polymer dispersion obtained by the process which is suitable for use as a retention and drainage aid in papermaking. The polymer dispersion is prepared by using an anionic polymeric stabilizer and an anionic polymeric co-stabilizer of relatively low molecular weight. The use of the polymeric stabilizer and co-stabilizer enables the use of from 0 to about 1.9 weight percent of inorganic salt. Preferably, the dispersion is substantially salt-free.
In general, aqueous dispersions of anionic polymer are easily producible and effective as retention and drainage aids in papermaking. However, there is still room for improvements. For instance, when preparing dispersion polymers in the presence of water-soluble salts, it has been experienced that high amounts of water-soluble salts are usually required to obtain effective and stable formation of polymer particles, which is undesired from an economic and environmental viewpoint. It has also been observed that the viscosity of the reaction medium and the obtained dispersion may be too high, which is undesired from a production and handling viewpoint. It has also been observed that dispersions of high content dispersion polymer are usually difficult to obtain, which is undesired from a transportation cost viewpoint. In addition, in respect of polymer dispersions containing more than one anionic stabilizer and, in particular, low molecular weight anionic co-stabilizers, it is known that low molecular weight anionic materials increase the cationic demand of cellulosic suspensions in papermaking and it has been experienced that low molecular weight anionic materials may interfere with and adversely effect the performance of cationic additives used in the papermaking process.
It would be advantageous to be able to provide an aqueous anionic polymer dispersion and process for its production which require a lower amount of water-soluble salt to achieve effective and stable polymer particle formation. It would also be advantageous to be able to provide an aqueous anionic polymer dispersion and process for its production which exhibit low viscosity and the avoidance of, or lower, viscosity peaks during the polymerization process. It would also be advantageous to be able to provide an aqueous anionic polymer dispersion and process for its production which enable higher dispersion polymer contents. It would also be advantageous to be able to provide an aqueous anionic polymer dispersion and process for its production which can be conducted without the use of low molecular weight anionic co-stabilizers. It would also be advantageous to be able to provide a papermaking process which comprises the use of aqueous anionic polymer dispersion exhibiting improved drainage and retention performance.